1. Field of the Invention
The present invention relates to an image forming apparatus that forms a color image and an endless belt drive controlling apparatus used in this image forming apparatus.
2. Description of the Related Art
Typical image forming methods for a color image forming apparatus are roughly classified to a direct transfer type and an intermediate transfer type. According to the direct transfer image forming method, toner images different in color and formed on a plurality of photoconductors, respectively are directly transferred onto a transfer sheet while registering the images on one another. According to the intermediate transfer image forming method, toner images different in color and formed on a plurality of photoconductors, respectively are transferred onto an intermediate transfer body while registering the images on one another. Thereafter, the images are collectively transferred onto a transfer sheet. Since such an image forming apparatus has the photoconductors arranged to face the transfer sheet or the intermediate transfer body, the apparatus is referred to as a “tandem image forming apparatus”. In the tandem image forming apparatus, an electrophotographic process including formation and development of an electrostatic latent image is executed for each color of yellow (Y), magenta (M), cyan (C), and black (K) per photoconductor. The images are transferred onto the transfer sheet that is being moved on a transfer and transport belt in the direct transfer type image forming apparatus. The images are transferred onto the intermediate transfer body that is being moved in the intermediate transfer type image forming apparatus.
For the tandem color image forming apparatus, it is important to highly accurately register the toner images in respective colors so as to prevent occurrence of out of color registration. For this reason, each of the direct transfer type apparatus and the intermediate transfer type apparatus is configured to attach an encoder to one of a plurality of driven rollers in a transfer unit. In addition, the apparatus of each type adopts a method for feedback controlling a rotational velocity of each driven roller according to a change in a rotational velocity of the encoder so as to avoid the out of color registration due to a change in a velocity of the transfer and transport belt.
The most common method for realizing a feedback control is a proportional control (PI control). The PI control is a method for controlling the belt so that an encoder output is always driven at the desired angular displacement. Specifically, in the PI control, a position error e(n) is computed from a difference between a desired angular displacement Ref(n) of the encoder and a detected angular displacement P(n−1) of the encoder. The position error e(n) thus computed is subjected to low pass filtering to eliminate high frequency noise, and multiplied by a control gain. A driving pulse frequency of a drive motor connected to a drive roller is controlled at a constant standard driving pulse frequency.
However, this PI method has the following disadvantages. If a thickness of the transfer and transport belt is changed slightly, a transport velocity of transporting the transfer sheet is changed. As a result, an image quality degradation that an image is deviated from a desired position and a fluctuation among images on a plurality of recording sheets, and a deterioration in a repeatability and a position reproducibility among the recording sheets occur.
Generally, a belt velocity, a radius of the driven roller, and a rotation angular displacement of the driven roller have a relationship as represented by the following equation. ω=V/r In the equation, ω denotes the rotation angular displacement, V denotes the belt velocity, and r denotes the radius of the driven roller.
In this relationship, it is known experientially that the radius r of the driven roller includes the thickness of the belt.
FIG. 18 is an enlarged view of a contact portion in which a roller 66 to which an encoder is attached (hereinafter, “encoder roller 66”) contacts with a transfer and transport belt 60. In FIG. 18, even if the transfer and transport belt 60 is moved at a constant velocity, an effective radius r of the encoder roller 66 is increased as long as a thick portion of the transfer and transport belt 60 is wound on the encoder roller 66. In addition, a rotation angular displacement of the encoder roller 66 per constant time is reduced. This reduction is detected as a reduction in a moving velocity of the transfer and transport belt 60. On the other hand, if a thin portion of the transfer and transport belt 60 is wound on the encoder roller 66, then the rotation angular displacement of the encoder roller 66 is increased, and the increase is detected as an increase in the moving velocity of the transfer and transport belt 60.
Due to this, even if the transfer and transport belt 60 is moved at a constant moving velocity, it is detected as if the moving velocity of the transfer and transport belt 60 is changed due to a change in belt thickness according to the rotation angular displacement detection by the encoder. In a driven shaft feedback control, this changed component is controlled to be amplified. This conversely adversely influences the belt moving velocity. As can be seen, the conventional feedback control method has a disadvantage in that a satisfactory feedback control in light of the change in belt thickness is not exercised.
As a method for solving a disadvantage of a feedback control failure resulting from the change in belt thickness, the following techniques are known as disclosed in, for example, Japanese Patent Application Laid-open (JP-A) Nos. 2000-310897, 2001-343878, and H11-126004. According to JP-A 2000-310897, if a drive roller is driven at a constant pulse rate, then a velocity profile is measured in advance so as to cancel a potential velocity change Vh that is generated due to a known thickness profile in all peripheral directions of the transfer and transport belt with reference to a position detected by a belt mark. A drive motor control signal is generated at a modulated pulse rate relative to the measured velocity profile. Based on this drive motor control signal, a motor is driven and the transfer and transport belt is driven through a drive motor. A final velocity Vb of the transfer and transport belt can be thereby made invariable.
JP-A No. 2001-343878 discloses an image forming apparatus that can start forming an image even before detection of a home position of a transfer and transport belt or an intermediate transfer belt, and that can reduce a time since the apparatus is activated until a first image is output. The image forming apparatus includes a movable belt member, an image forming unit that forms an image on the belt member or a recording material carried by the belt member, a detector, and a storage unit. The detector detects a reference position of the belt member. The storage unit stores information representing a movement amount by which the belt member is moved after the detector detects the reference position of the belt member when the belt member is stopped.
JP-A No. H11-126004 discloses an image forming apparatus that can detect an average velocity change throughout a belt without nipping the belt. The image forming apparatus includes a plurality of belt transport rollers including a belt drive roller and a velocity detection roller, a belt supported by the rollers, and a belt velocity controller. The velocity detection roller is arranged to be apart from the belt drive roller by a distance equal to or larger than a quarter of a perimeter of the belt. The belt velocity controller includes a roller rotational velocity detection sensor, a roller drive motor, a motor drive circuit, and a motor drive signal output unit. The roller rotational velocity detection sensor detects a rotational velocity of the velocity detection roller. The roller drive motor drives the belt drive roller to be rotated. The motor drive circuit drives the roller drive motor. The motor drive signal output unit outputs a motor drive circuit control signal according to a detection signal of the roller rotational velocity detection sensor.
However, these conventional techniques have the following disadvantage. The feedback control in light of the change in the belt moving velocity generated due to the thickness change of the endless belt cannot be exercised stably and favorably according to an image quality. In addition, the thickness of the endless belt spread over the rollers is changed, depending on a position at which the belt is left stopped, a belt leaving time, or the like. However, a technique for feedback controlling the endless belt in light of the thickness change of the belt is not developed yet.